1. Field of the Invention
Using processes for producing a directionally solidified casting, it is possible to produce components of a complex design which can be subjected to high thermal and mechanical stresses, such as guide vanes or rotor blades of gas turbines. Depending on the processing conditions, the directionally solidified casting can in these cases be designed as a monocrystal or be formed by columnar crystals which are aligned in a preferred direction. It is of particular importance that the directional solidification takes place under conditions in which a high level of heat exchange takes place between a cooled part of a casting mold which receives molten starting material and the starting material which is still molten. A zone of directionally solidified material can then develop, having a solidification front which migrates through the casting mold under continuing removal of heat, forming the directionally solidified casting.
The production of a sound casting depends essentially on the magnitude of the temperature gradient at the solidification front and on the rate of solidification. With a low temperature gradient and a high rate of solidification, it is not possible to produce a directionally solidified casting. By contrast, with a high temperature gradient and a low rate of solidification, it is in fact possible to produce a directionally solidified casting, but such a casting has unwanted defects, such as in particular chains of equiaxed grains (freckles).
2. Discussion of Background
The invention proceeds from a process for producing a directionally solidified casting and from an apparatus for carrying out the process as is described, for example, in U.S. Pat. No. 3,532,155. The process described serves to produce the guide vanes and rotor blades of gas turbines and using a vacuum furnace. This furnace has two chambers which are separated from one another by a water-cooled baffle and are arranged one above the other, the upper chamber of which is designed so that it can be heated and has a pivotable melting crucible for receiving material to be cast, for example a nickel base alloy. The lower chamber, which is connected to this heating chamber by an opening in the water-cooled baffle, is designed so that it can be cooled and has walls through which water flows. A driving rod which passes through the bottom of this cooling chamber and through the opening in the water-cooled baffle bears a cooling plate through which water flows and which forms the base of a casting mold located in the heating chamber.
When carrying out the process, first of all an alloy which has been liquefied in the melting crucible is poured into the casting mold located in the heating chamber. A narrow zone of directionally solidified alloy is thus formed above the cooling plate forming the base of the mold. As the casting mold is moved downward into the cooling chamber, this mold is guided through the opening provided in the water-cooled baffle. A solidification front which delimits the zone of directionally solidified alloy migrates from the bottom upward through the entire casting mold, forming a directionally solidified casting.
At the start of the solidification process, a high temperature gradient and a high rate of solidification are achieved, since the material which is poured into the mould initially strikes the cooling plate directly and the heat which is to be removed from the melt is led from the solidification front through a comparatively thin layer of solidified material, with a heat transfer coefficient .alpha..sub.cm, to the cooling plate. If the material has a relatively low coefficient of thermal conductivity, as the distance between the cooling plate and the solidification front increases, heat is increasingly dissipated through the walls of the casting mold, with a heat transfer coefficient .alpha..sub.cmd, and also radiated from the mold surface, with a heat transfer coefficient .alpha..sub.r, into the cooler environment. In accordance with Newton's law of cooling, the heat q removed from the casting is then determined as follows: EQU q=.alpha.(T-T.sub.o),
where T is the average temperature of the casting and T.sub.o is the ambient temperature, as it is determined, for instance, by the water-cooled walls of the cooling chamber, and where 1/.alpha.=1/.alpha..sub.cm +1/.alpha..sub.cmd +1/.alpha..sub.r.
For a large gas turbine blade made of a nickel base superalloy, the following values of the heat transfer coefficients are typically found: EQU .alpha..sub.cm =lambda.sub.m /.delta..sub.m =816 J/m.sup.2 sK, EQU .alpha..sub.cmd =lambda.sub.md /.delta.md=200 J/m.sup.2 sK,
where lambda.sub.m and lambda.sub.md are the coefficients of thermal conductivity of the alloy and of the ceramic casting mold, respectively, and .delta..sub.m and .delta..sub.md are the thickness of the layer of metal which has already solidified (taken as 30 mm) between the part of the mold wall situated below the water-cooled wall and the solidification front and the thickness of the mold wall (taken as 10 mm), respectively, and .alpha..sub.r =.sigma.(.epsilon..sub.1 T.sub.1.sup.4 -.epsilon..sub.2 T.sub.0.sup.4)/(T.sub.1 -T.sub.0)=130 J/m.sup.2 sK, where .sigma. is the Stefan-Boltzmann constant, .epsilon..sub.1, T.sub.1 and .epsilon..sub.2, T.sub.0 are the emission capability and temperature of the casting mold surface and the absorption capability and temperature of the environment, respectively, (.epsilon..sub.1 =.epsilon..sub.2 =0.5; T.sub.1 =1500K; T.sub.0 =400K).
This gives .alpha.=72 J/m.sup.2 sK.
A further process for producing a directionally solidified casting is disclosed in U.S. Pat. No. 3,763,926. In this process, a casting mold filled with a molten alloy is gradually and continuously immersed into a tin bath heated to approximately 260.degree. C. This achieves a particularly rapid removal of heat from the casting mold. The directionally solidified casting formed by this process is distinguished by a microstructure which has a low level of inhomogeneities. When producing gas turbine blades of comparable design, it is possible using this process to achieve a vales which are almost twice as high as when using the process according to U.S. Pat. No. 3,532,155. However, in order to avoid unwanted gas-forming reactions, which can damage the apparatus used in carrying out this process, this process requires a particularly accurate temperature control. In addition, the wall thickness of the casting mold has to be made larger than in the process according to U.S. Pat. No. 3,532,155.